It is well known that different kinds of filters or filtering circuits are constantly employed in various types of electronic equipment which is used in computer, telemetering, radar, radio, television, and other nonvital systems. Normally, these previous filter circuits are quite acceptable for use in such nonvital applications but are wholly unsatisfactory for employment in specific vital operations. For example, in signal and control systems for railroad and/or mass and rapid transit operations, it is essential and, in most cases, mandatory for ensuring that specific portions or circuits function in a vital or fail-safe manner. These vital circuits must not simulate a safe condition during the failure of a critical circuit component in order to provide the highest degree of safety to individuals and to minimize the chance of damage to apparatus. Thus, reliability and safety are the two most important factors in obtaining acceptance and approval for vital vehicular control systems. That is, such stringent operating requirements are necessary in order to prevent costly damage to the equipment as well as to preclude serious injury and possible death to employees and passengers. In a coded type of vehicle control system, the filtering circuit must only pass or respond to a selected center frequency or carrier signal which is transmitted into a given block or section of railroad track. Thus, the filtering circuit operates as a narrow band-pass filter which is capable of passing a preselected modulated carrier and serves to attenuate or reject spurious noise and adjacent channel carrier signals. It will be appreciated that a vital electronic filter circuit must always attenuate noise and other carrier signals even during the presence of an unsafe circuit or component failure.